In PADM (Per transmit Antenna Differential Mapping) transmission diversity, a differential modulation different for each transmit antenna is performed on identical data, and these data are transmitted. For example, by delaying the transmission timing of data to be transmitted via one of two antennas by one symbol and using an equalizer on a receive side, high resistance to variations in the transmission line can be achieved. The PADM transmission diversity is also referred to as a PADE (Per transmit Antenna Differential Encoding) method.
Currently, because modulation methods respectively used for antennas are the same as each other in a transmission time diversity system which is used typically, a blockage of one of the transmit antennas causes an occurrence of uncertainty in the timing and hence causes an error. The PADM is a method which has been proposed in order to solve this problem, and is a time diversity method of applying a different differential encoding mapping to each transmit antenna. When carrying out communications using the PADM, a transmit side performs creation of a modulated signal using the PADM, and a receive side executes a demodulation method using PSP (Per-Survivor Processing). Because demodulation characteristics degrade if a large frequency error occurs during transmission and reception when the receive side performs a demodulation using PSP, the receive side estimates the frequency error and corrects the frequency error on the basis of the estimation result, and then performs the PSP demodulation.
Patent reference 1 discloses a transmitter which, when transmitting signals created on the basis of an identical information sequence by using a single carrier from two transmit antennas, performs a different modulation process on each of encoded sequences respectively corresponding to the above-mentioned two antennas.
Furthermore, patent reference 2 discloses a receiver which estimates a frequency error of a received signal in a communication system which uses the PADM transmission diversity and carries out a demodulation process using PSP (Per-Survivor Processing).